A turbocharger is configured to rotary-drive a turbine with exhaust gas of an engine, compress air through rotation of a compressor provided coaxially with the turbine, and supply the compressed air to the engine, and is capable of increasing the output of the engine or reduce the fuel consumption amount at the same output.
The work of the turbocharger depends on energy that the turbine receives from exhaust gas. Thus, in an engine low rotation speed region where the amount of exhaust gas is small, the supercharging effect is low.
As one of the configurations capable of solving the above problem, a multi-stage turbo supercharging system with a plurality of turbochargers is known. For instance, a two-stage turbocharger including combination of a larger turbocharger and a smaller turbocharger has been commercialized.
A two-stage turbocharger may include two larger and smaller turbochargers, a bypass passage that bypasses the smaller turbocharger, and a bypass valve disposed in the bypass passage.
With this configuration, when the engine is operating at a low rotation speed and a low load and the flow rate of exhaust gas is low, both of the bypass valves on the turbine side and the compressor side close and exhaust gas is fed to the smaller turbocharger, which makes it possible to perform efficient supercharging with a small flow rate of exhaust gas. On the other hand, when the engine is operating at a high rotation speed and the flow rate of exhaust gas is sufficient, both of the bypass valves on the turbine side and the compressor side open and exhaust gas is fed to the larger turbocharger bypassing the smaller turbocharger, which makes it possible to address a high flow rate of exhaust gas.
Patent Document 1 discloses a multi-stage turbo supercharging system including a high-pressure stage with a smaller rotor diameter, a low-pressure stage with a larger rotor diameter, and a bypass passage that bypasses the turbine of the high-pressure stage in the exhaust passage of the engine.
Patent Document 2 discloses a multi-stage turbo supercharging system including a high-pressure stage with a smaller rotor diameter, a low-pressure stage with a larger rotor diameter, a bypass passage that bypasses the turbine of the high-pressure stage in the exhaust passage of the engine, and a bypass passage that bypasses the turbine of the low-pressure stage in the exhaust passage of the engine.